since和过去完成时能否搭配
问题描述：
since和过去完成时能否搭配
依照定义,since应该是不可以和过去完成时搭配的,但是《星火英语语法全解》上写“常与过去完成时连用的时间状语”中包括了'since he left'.但是又没给例句.求讲解.
答
since的意思是自从,不能与完成时态连用,一般用法是1.主句完成时,从句since+时间点2.主句完成时,从句since+过去时态3.It has been+时间段,since+过去时态例如：I haven't heard from him since last year.我自去年以...能用，刚刚查了下资料，since是可以与现在完成时态和过去完成时态连用的。
since与现在完成时态连用，例如：
Since I have been in this class, we have had three English teachers.自从我来到这个班，，我们已经换了三个英语老师了。
Since I was in this class,we have had three three English teachers.自从我离开这个班，我们已经换了三个英语老师了。
since与过去完成时态连用（一般都是在间接引语中）例如：
He said it was 2 years since he had smoked.他两年没抽烟了。(要翻译成否定的含义）
said之后的宾语从句本来是现在完成时态，由于said是过去时态，根据间接引语的时态变换规则，从句要变成过去完成时。